Amazon tablets run their own brand of Android with their own app store, do they not? Maybe things have changed since the original Fire tablets launched. I don't know how dominant those tabs are outside the US. In any case, I didn't mention Amazon as the Fire line these days is more of a way to lock people into their ecosystem of Prime video and music, rather than a "do whatever you want" style tablet.
It is possible to install the Google App Store on a Fire, and does not require rooting it, but it is a pain and most people wouldn't bother.
I agree that they really are just to lock people into the Amazon ecosystem.